Any quarry Aggregate Industries ends up digging at Straitgate Farm must not introduce water bodies, because of aircraft safeguarding considerations at Exeter Airport:
But Aggregate industries is not very good at controlling surface water.
If it were, it would not be suffering a "surface water ponding issue" at its Hillhead Quarry near Uffculme. We’ve already posted about this – here, here, here and here.
Aggregate Industries has recently lodged a planning application with Devon County Council for Hillhead Quarry near Uffculme – DCC/4399/2024:
Variation of conditions 2, 4, 6, 7, 12, 19, 22 and 25 of ROMP permission DCC/3655/2014... to vary the phasing; landform; drainage; and restoration of the site.
The Planning Statement for the application reads:
3.4.1 Houndaller Extension Area is currently facing surface water ponding issues in Phase 6 which is preventing completion of restoration of this section of the quarry. It is considered that a similar situation will occur once extraction operations cease in Phase 7. During 2023 and to date in 2024, in excess of 1m dept [sic] of water has collected over much of the Phase 6 area. The consented final restoration landform does not allow for effective drainage. Therefore, Aggregate Industries are proposing a long-term solution for surface water ponding issues on site.
Just how wrong has Aggregate industries got things at Hillhead?
There may be in excess of 1m depth of water, but how far does that water extend? Fortunately, Google Earth has recently provided new imagery for the area – and the answer is 9.27 acres or 3.75 ha, giving a volume of water in excess of 37,500 cubic metres, or enough to fill 15 Olympic-sized swimming pools.
Where is all this water to go, now and in the future? According to the Hydrological & Hydrogeological Impact Assessment for the above application:
5.8.8.1 ... ingress waters (rainfall runoff and groundwater seepage) will be captured by the perimeter drainage ditch and directed into Houndaller Plantation Pond, which will recharge the groundwater system in the BSP Aquifer.5.8.8.2 An infiltration rate of 50 m/d has been used in the calculations, which is the worst-case value for hydraulic conductivity derived from falling head tests (as used for calculating the worst-case ingress rates in the extraction area).5.8.8.3 The design storm would raise the water level (injection head) in the pond by 0.5 m.5.8.8.4 The mean surface area (through which the outflow occurs in the recharge / soakaway feature) is taken to be the sides of the pond when accommodating 0.5 m change in water level.5.8.8.5 The unlined flanks of the pond have a total length of 400 m and have an effective depth of 0.5 m; therefore, the total surface area for infiltration would be 200 m2.5.8.8.6 Based on Darcy’s Law, applying an injection head of 0.5 m and an infiltration rate of 50 m/d, and with the mean surface area for infiltration set at 200 m2, the soakaway rate (recharging the BSP Aquifer) equates to 5,000 m3/d.5.8.8.7 This is equivalent to circa 150% of the storm ingress ASV indicated at section 5.8.7.14. Therefore, it is concluded that Houndaller Plantation Pond has sufficient soakaway capacity for the design storm.
However, this calculation doesn’t look right: a soakaway rate of 5000 cubic metres of water per day through an infiltration area of just 200 square metres seems impossibly large.
What do we find? Firstly, the author of the report has not assumed "the worst-case value for hydraulic conductivity" they have in fact assumed the best:
3.5.9.1 The hydraulic conductivity of the BSP has been determined from falling head tests undertaken in the area; and typically varies between 5.8x10-4 and 7.3x10-6 m/s, equivalent to 0.6-50 m/d.
Secondly, the author has omitted the length of the flow path (l) from their calculations. Darcy’s Law relates the flow of groundwater through the saturated aquifer (Q) to the cross section of the aquifer (A) and the hydraulic gradient (h/l) and is written as Q = -kAh/l, where k is the hydraulic conductivity.
Using the worst case of 0.6m/d for hydraulic conductivity (k), with 200m2 (A), 0.5m (h) and, for example, a 1000m flow path (l), the soakaway rate would fall from the incredible 5,000 m3/d to just 0.6 x 200 x 0.5/1000 = 0.06m3/day – almost 100,000 times less.
Which means that it’s not at all clear that Houndaller Plantation Pond does have "sufficient soakaway capacity for the design storm."
The Hydrological & Hydrogeological Impact Assessment for the above application was authored by the same team that will, either this month or next*, be trying to work out infiltration rates at Straitgate Farm. Hardly instills confidence does it?
EDIT 25.6.24 *Infiltration tests now postponed by Aggregate Industries until "end of July/early August".
EDIT 12.7.24 Objection from Devon County Council Flood Risk Team
EDIT 2.8.24 Regulation 25 Request
